1. Field of the Invention
The present invention generally relates to an electrophotographic image forming apparatus such as, for example, a copying machine and a printer. More particularly, the present invention relates to the electrophotographic image forming apparatus wherein an electrostatic latent image is formed on a surface of an electrostatic latent image carrier supported for movement in one direction after the carrier surface has been electrostatically charged by a stationary brush charger and has subsequently been exposed to imagewise rays of light, and is then developed into a visible powder image which is in turn transferred onto a recording medium.
2. Description of the Prior Art
In the well-known electrophotographic image forming apparatus such as, for example, an electrophotographic copying machine or an electrophotographic printer, a surface of an electrostatic latent image carrier is electrostatically charged prior to the formation of an electrostatic latent image thereon by exposure to imagewise rays of light. The electrostatic latent image is subsequently developed into a visible powder image which is in turn transferred onto a recording sheet. The powder image transferred onto the recording sheet is permanently fixed thereon to provide a copy as the recording sheet bearing the powder image is transported through a fixing unit.
As an electrostatic charger for developing the electrostatic charge on the surface of the latent image carrier, a corona charger and a contact brush charger are well known to those skilled in the art. Of them, the contact brush charger has recently received much attention partly because the contact brush charger would not deteriorate the carrier surface and partly because, as compared with the corona charger, the contact brush charger generates a minimized quantity of ozone which is generally recognized toxic to human being. While the contact brush charger is available in various models, the stationary contact brush charger which is supported fixedly relative to the movable latent image carrier has gained wide acceptable because of a simplified structure as compared with the rotary contact brush charger which is supported for rotation relative to the latent image carrier.
The contact brush charger, however, has its own peculiar problem in that the potential of the electrostatic charge built up on the surface of the latent image carrier tends to vary from place to place over the surface of the latent image carrier, constituting a cause of an appearance of line noises on the resultant image bearing copy.
Various attempts have hitherto been made to avoid the variation in potential of the electrostatic charge built up on the surface of the latent image carrier. For example, the Japanese Laid-open Patent Publication No. 64-23266, published Jan. 25, 1989, addresses to the contact time over which any arbitrarily chosen point on the surface of the latent image carrier is held in contact with an electroconductive brush forming the stationary contact brush charger. According to this publication, in order to avoid the varying potential of the electrostatic charge built up on the surface of the latent image carrier, the width of the electroconductive brush as measured in a direction transverse to the direction of movement of the surface of the latent image carrier and in a direction conforming to the widthwise direction of the latent image carrier is so chosen that any arbitrarily chosen point on the surface of the latent image carrier being moved contacts the electroconductive brush for a predetermined contact time not shorter than 0.1 second.
We have, however, found that, where the surface of the latent image carrier is electrostatically charged by the stationary contact brush charger, the electrostatic charging is accomplished through three closely related mechanisms of discharge, injection and friction during the contact taking place between the stationary contact brush charger and the surface of the latent image carrier. Of these mechanisms, the friction participates in the electrostatic charging to a negligible extent, but the discharge and the injection participate to a large extent. The electrostatic charging by discharge and injection is affected not only by the contact time, i.e., the duration of contact, between the electroconductive brush of the stationary contact brush charger and any arbitrarily chosen point on the surface of the latent image carrier, but also by the electric resistance exhibited by the electroconductive brush. This will now be discussed in further detail.
In the first place, how the contact time t affects the electrostatic charging by discharge and injection will be discussed. If the electroconductive brush forming a part of the stationary contact brush charger has a relatively small width and/or if the velocity of movement of the latent image carrier is high, the contact time t is naturally short. Referring now to FIG. 14, if the stationary contact brush charger has a relatively small width WD, brush bristles F are apt to break up, to form biases or to be disordered in any way during the image forming process to such an extent as to eventually result in a varying distribution of the electrostatic charge built up on the surface of the latent image carrier. This is also true even where the width WD is relatively large, provided that the velocity of movement of the surface of the latent image carrier is relatively high. An occurrence of such a disorder of the brush bristles F is considerable with an increase in number of copies or prints having been made.
On the other hand, if the stationary contact brush charger has a relatively large width and/or if the velocity of movement of the surface of the latent image carrier is relatively low, the contact time t is naturally long. Too long contact time t tends to result in a smudging of the brush bristles F such as adherence of toner fragments, particles ground off from the latent image carrier and/or paper dust to the brush bristles F, particularly tips thereof, as shown in FIG. 15 and, in the worst case it may occur, the bristle tips would entirely be covered up. The brush bristles F are similarly smudged locally when a cleaning unit used to remove residue toner from the surface of the latent image carrier is distorted.
In any event, such a smudging of the brush bristles lowers the efficiency of charge injection accomplished by the stationary contact brush charger. Once this occurs, a relatively large difference in amount of electrostatic charge built up on the latent image carrier is created between a portion of the surface of the latent image carrier, which has been held in contact with a smudged portion of the brush bristles where the efficiency of charge injection was lowered, and a less smudged portion of the surface of the latent image carrier which has been held in contact with a portion of the brush bristles where the efficiency of charge injection was high, resulting in a varying distribution of the electrostatic charge on the surface of the latent image carrier.
By the reason discussed above, it does not appear that the use of the long contact time between the contact brush charger and the surface of the latent image carrier such as suggested in the Japanese Laid-open Patent Publication No. 64-23266 is an effective solution to eliminate the varying distribution of the electrostatic charge on the surface of the latent image carrier. A combination of the width of the stationary contact brush charger and the velocity of movement of the surface of the latent image carrier, that is, the magnitude of the contact time t between the surface of the latent image carrier and the stationary contact brush charger, affects the pattern of distribution of the electrostatic charge built up on the surface of the latent image carrier.
We will now discuss how the electric resistance (hereinafter referred to as bristle electric resistance) of the brush bristles affect the pattern of distribution of the electrostatic charge on the surface of the latent image carrier. If the electric resistance R of each of the brush bristles, that is, the bristle electric resistance R, is too low, the resistance of the brush bristles F as a whole increases as the brush bristles F are smudged, and the increased resistance considerably affects the characteristic of electrostatic charge built up according to Paschen's law. For this reason, that portion of the surface of the latent image carrier which has been held in contact with the smudged portion of the brush bristles exhibits a lower surface potential than that exhibited by the portion thereof which has been held in contact with the less smudged portion of the brush bristles, resulting in a varying pattern of distribution of the electrostatic charge on the surface of the latent image carrier.
On the other hand, if the bristle electric resistance R is too high, a varying resistance at different portions of the brush bristles brings about an unnegligible influence on the built-up of the electrostatic charge by discharge. More specifically, even though the brush bristles are rated to have a volume resistivity of, for example, 10.sup.6 .OMEGA./cm, manufacturing circumstances makes the actual volume resistivity vary from 10.sup.5 to 10.sup.7 .OMEGA./cm. Since the stationary contact brush charger includes the contact brush having a flock of some hundred thousand bristles, the variation in volume resistivity cannot be negligible. Specifically, at a region of the contact brush having a relatively high bristle electric resistance R, the variation of the volume resistivity in a magnitude of plus or minus one figure with respect to the rated value acts considerably on a voltage drop, resulting in that respective portions of the contact brush having the relatively high and low bristle electric resistance R tend to be charged to high and low values, respectively. This in turn result in a varying pattern of distribution of the electrostatic charge built up on the surface of the latent image carrier.
As hereinabove discussed, the electrostatic charging by discharge and injection depends on the contact time t and the bristle electric resistance R of each bristle forming the brush of the stationary contact brush charger and, therefore, unless a proper value is chosen for each of the contact time t and the bristle electric resistance R, the surface of the latent image carrier cannot be electrostatically charged to a potential enough to avoid a varying distribution of the electrostatic charge thereon. Also, where the environment dependency (for example, the dependency on temperature and/or moisture) after the image forming which has taken place for a long time is desired to be minimized, the contact time t and the bristle electric resistance R should receive much attention.